The present technology relates to semiconductors.
Most semiconductor devices include an input circuit configured to receive signals from the outside world via input pads (or pins) and an output circuit configured to provide internal signals to the outside via output pads (or pins). The input circuit has a termination resistor for impedance matching of the external transmission line. The output circuit has an output driver, which has a resistance (Rory). The impedance of both the termination resistor and the output driver can change due to various circumstances such as variation of a power supply voltage, a change in operating temperature, etc. Thus, an impedance mismatch can arise.
The impedance mismatch can cause signal reflection, which can compromise signal integrity. As the operating speed of electrical products has increased, the swing width (that is the difference between high and low logic levels) of a signal interfaced between semiconductor devices gradually has been reduced in order to reduce supply current and to minimize the delay time taken for signal transmission. However, the reduction in the swing width of the signal easily exposes the signal to external noise, causing signal reflection to become more critical at an interface terminal due to impedance mismatch. Thus, the impedance mismatch may lead to difficulty in high-speed transmission of data and distortion of output data.
Weak output drivers may result in bad slopes on I/O signals. Strong output drivers may consume high currents and result in too much simultaneous switching noise (SSN) on power lines and I/O pins. Mismatch in pull up and pull down impedances also degrades signal integrity. Therefore, at high speeds, any deviation in output driver impedance from target impedance results in shrinking of eye aperture and may result in data transmission errors or losses.
To alleviate impedance mismatch, the device's output drivers and/or the device's termination resistors may be periodically calibrated. One calibration technique is referred to as a ZQ calibration. Conventionally, ZQ calibration may use a precision calibration resistor that is located off chip. The chip may have a variable impedance circuit which is calibrated with respect to the off chip resistor. The ZQ calibration adjusts the impedance of the variable impedance circuit until it is calibrated to the off chip resistor. This results in an impedance code that can be used to adjust the impedance of the device's output drivers or on-die termination circuitry.